The present invention relates to method and a system for reading a radiation image that has been stored in a photostimulable phosphor screen. More particularly the invention relates to the re-use of the photostimulable phosphor screen.
Radiation image recording systems wherein a radiation image is recorded on a photostimulable phosphor screen by exposing said screen to image-wise modulated penetrating radiation are widely used nowadays.
The recorded image is reproduced by stimulating the exposed photostimulable phosphor screen by means of stimulating radiation and by detecting the light that is emitted by the phosphor screen upon stimulation and converting the detected light into an electrical signal representation of the radiation image.
In such a system it is preferred, in view of economy, that the stimulable phosphor screen can be used in many imaging cycles.
The reuse of the stimulable phosphor screen is possible when the previously stored radiation image is erased to a sufficient extent.
When reading out an image by stimulating a phosphor screen that has been exposed to penetrating radiation, less than 90% of the stored energy is released. Thus there arises a problem that, upon reuse, part of the radiation image is still stored in the phosphor screen and can appear in the subsequent image as a so-called ghost image.
In general medical radiography, images are made with widely differing X-ray doses.
To make images of extremities, like e.g. fingers, doses are used of the order of 1 mR. On the other hand, images of internal organs, like the stomach are made with X-ray doses that may be as high as 300 mR.
To avoid ghosting, when making a 1 mR image immediately after a 300 mR image, the signal of the first image must be reduced by more than a factor of 300.
As a matter of fact, a dynamic range is desired in the second image of at least 100. This implies that the signal created by the first irradiation must be reduced by a factor of at least 3.104, which is equivalent to requiring an erasure depth of 1/(3.104)=3.3.10xe2x88x925.
According to U.S. Pat. No. 3,859,527 (column 4, lines 5-7) the phosphor can be reduced to neutral state by actions like a uniform illumination, irradiation or heating.
In commercial systems, the phosphor screen is erased by illumination with visible light. Incandescent lamps are commonly used because they are cheap, high power light sources.
High power is needed, because in order to guarantee a high through-put scanning system, the phosphor screen must be erased in a short time.
In the first place, high power lamps generate a lot of heat, which may destabilise the scanner to read out the storage phosphor screens. The size of the read out apparatus has to be rather large in order to enable removal of the heat generated by a powerful erasure unit which is required in order to have a high throughput. The more compact the read out apparatus, the more difficult it will be to remove the heat generated by a powerful erasure unit.
In the second place, an erasure unit containing incandescent lamps, like e.g. quartz halogen lamps, will be rather bulky because of the sockets that are needed to make a light source that illuminates the phosphor screen homogeneously.
In order to develop a reliable and compact storage phosphor screen digital radiography system it is important to reduce the power consumed by the erasure unit. This was not possible in the state of the art system without negatively affecting the throughput.
It is an object of the present invention to provide a method and a system for reading a radiation image that has been stored in a photostimulable phosphor screen wherein the screen is erased in between successive recordings to an adequate extent so as to permit re-use of the screen.
It is a further object of the present invention to provide such a system that is compact and has at the same time a high throughput.
Further objects will become apparent from the description given below.
The above mentioned objects are realised by a method having the specific features defined in claim 1.
Another aspect of the present invention relates to an apparatus for reading a radiation image that has been stored in a phosphor screen comprising a divalent europium activated cesium halide phosphor wherein said halide is at least one of chloride and bromide. The apparatus comprises a source of stimulating radiation arranged for emitting stimulating light, means for directing said stimulating light onto a photostimulable phosphor screen, a transducer for converting light emitted by said phosphor screen upon stimulation into an electrical signal and an erasing unit for erasing said photostimulable phosphor screen after having been stimulated. The erasing unit comprising at least one laser.
The term xe2x80x98an erasing light source assemblyxe2x80x99 refers to either a single erasing light source or a group of more than one erasing light source. In the latter case when the electrical power is specified the value refers to the electrical power of the total assembly.
The present invention enables the use of a compact and low power erasure assembly without the need to increase the erasure time to an unacceptable extent. In this way the read out apparatus can be made very compact while its throughput is not negatively affected.
Still another aspect relates to a re-usable radiation detector comprising
a photostimulable phosphor screen,
at least one source of stimulating light arranged for stimulating said phosphor screen,
an array of transducer elements arranged for capturing light emitted by the phosphor screen upon stimulation and for converting said light into an electrical signal representation,
an erasing unit arranged for erasing said screen when it has been read out,
means for transporting the phosphor screen and an assembly of stimulating light source(s), said array of transducer elements and said erasing unit relative to each other,
an enclosure enclosing said photostimulable phosphor screen, said stimulating light source, said erasing unit, said array of transducer elements and said means for transporting the screen,
interfacing means for communicating said electrical representation to an external signal processing device, wherein said erasing unit comprises at least one laser.
In one embodiment said stimulating light source and said erasing laser is the same light source.
Specific features for preferred embodiments of the invention are set out in the dependent claims.
In this document the term xe2x80x9cradiationxe2x80x9d has to be understood as any penetrating radiation and includes irradiation originating from a radioisotope (e.g. Co60, Ir192, Se75, etc.), radiation created by an X-ray generator of any type, radiation and high energy particles created by a high energy radiation generator (e.g. Betatron), radiation from a sample labeled with a radioisotope as is the case in e.g. autoradiography.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.